1. Field of the Invention
The present invention relates to image forming apparatuses such as a copying machine, a facsimile machine, and a printer, as well as to a sheet feeding device for use in the image forming apparatuses.
2. Description of the Related Art
Heretofore, there have been known sheet feeders as disclosed in Japanese Examined Utility Model Publication No. 07-14271 and Japanese Patent No. 2614256 for use in an image forming apparatus. The sheet feeders are used for a so-called manual tray. Normally, sheet cassettes each adapted to accommodate a plurality of recording sheets of a predetermined size are detachably mounted on a main body of an image forming apparatus so that the recording sheets of the predetermined size e.g. A4 format or B5 format, which are accommodated in the respective sheet cassettes, are automatically fed to a transferring section. In the case where the sheet cassettes do not accommodate the recording sheets to be fed to the transferring section, a manual tray is used. In use of the manual tray, a user directly places a recording sheet or sheets to be fed to the transferring section on the manual tray without using the sheet cassettes.
The sheet feeders recited in the above publications are designed on a premise that a plurality of recording sheets are placed on the manual tray. Accordingly, although the sheet feeders are efficiently operable in a case that a plurality of recording sheets are placed on the manual tray, the sheet feeders are sometimes inoperable in a case that a single sheet to be fed is placed on the manual tray. In particular, it is often the case that a single recording sheet to be fed from the manual tray has a larger or a smaller thickness as compared with an ordinary recording sheet of the predetermined format size. Accordingly, it is difficult for the conventional sheet feeder to securely feed the single recording sheet having such a larger or smaller thickness to the transferring section.